The present invention comprises a new dwarfing rootstock for pears (Pyrus communis) referred to by the varietal name `Rhenus 1`. The federally registered trademark `Pyrodwarf` is the brand name for marketing this plant in the United States.
Very few dwarfing rootstocks for pears are currently available. Clones of quince (Cydonia oblonga) are used by pear growers in maritime countries of northern Europe to provide vigor control, scion precocity, and good yield efficiency. However, quince is incompatible with many commercial pear varieties, including `Bartlett`. Moreover, since most quince clones exhibit poor tolerance to winter cold, growers in areas with cold winters must employ Pyrus species rootstocks, most of which are invigorating.
Although selections from the cross between the pear varieties `Old Home` (unpatented, believed to be publicly available) and `Farmingdale` (OHxF) (unpatented, believed to be publicly available) have been investigated as rootstocks for pear varieties, none appears to have significant size-controlling ability, particularly when grown on deep loam or clay soils with good moisture supply.
Pyrus rootstocks are usually extremely difficult to propagate by conventional vegetative methods. As a result, about one-half are propagated from seed. They also often stimulate poor scion precocity and cause smaller fruit size.
The new variety was selected by me in a cultivated area in a orchard at the Research Station of Viticulture and Horticulture in Geisenheim, Germany. Pear varieties `Old Home` (female) and `Bonne Louise d'Avranches` (male) (unpatented, believed to be publicly available) were cross-pollinated and 2500 of the resulting seeds were germinated. In a first evaluation, two pear varieties, `Bartlett` and `Doyenne de Comice` were grafted to the seedlings. The grafted seedlings then were evaluated as rootstocks according to the following criteria: trunk cross-sectional area, cumulative marketable yield, yield efficiency, blossoming, tree health, compatibility, frost hardiness, suckering, soil adaptation. Twenty-two seedling clones that displayed the best results were selected for further testing. The selected seedling clones were cut below the grafting union to produce root suckers for asexual reproduction and virus indexing. `Rhenus 1` was selected from among the twenty-two clones as giving the best results.
`Rhenus 1` has not been observed under all possible environmental conditions and its phenotype may vary significantly with variations in environment such as temperature, light intensity, and day length, without any variation in genotype. However, the following unique combination of characteristics relating to the use of `Rhenus 1` as a rootstock for pear varieties, distinquish `Rhenus 1` from all other pear varieties of which I am aware: (1) causes dwarfing of pear varieties to a degree at least as great as quince A rootstock without significantly reducing fruit size; (2) compatible as a rootstock with all pear varieties tested; (3) high yield efficiency; (4) early yield (precocity); (5) substantially uniform fruit size; (6) high frost hardiness; (7) no suckering; (8) and good soil adaptation. These characteristics are established and transmitted through succeeding asexual propagations.
Asexual reproduction of `Rhenus 1` is accomplished, for example, by tissue culture; softwood and hardwood cuttings, e.g., using 2000 ppm indole-3-butyric acid (IBA) (quick dipping) by the "methode Trefoise" (R. Trefoise, "Description des techniques de bouturage feuille des nouveaux sujets porte-greffe nanifiants dur cerisier," Note technique du Centre de Recherches Agronomiquies de l'Etat Gembloux, No. 10/50, Septembre 1988, I.S.S.N. 0771-0607, 1988); and by stool bed propagation (layering).